1. Field of the Invention
The present invention relates to a laminated thin film, a phase plate, and a display apparatus that projects an image displayed by a reflection type liquid crystal device.
2. Description of the Related Art
There is known a reflective liquid crystal display apparatus including a light source portion, an optical system for separating and combining an illumination light ray from the light source in accordance with its polarization, a reflection type liquid crystal display device for converting an emitted light ray into an image light ray, and a projection optical system for imaging the converted image light ray.
Hereinafter, a typical structure thereof is described. White color light emitted from a light source is aligned to have a predetermined polarization direction by a polarization conversion device and is separated into green, blue, and red color band light rays by a dichroic mirror. The blue and red color band light rays are further processed by a color selective phase plate to have a polarization direction of a predetermined wavelength band converted by 90 degrees, and illuminate liquid crystal display devices corresponding to predetermined colors via a polarization beam splitter. The liquid crystal display device converts the illumination light into image light in accordance with an image signal and reflects the image light. The image light rays are combined by the polarization beam splitter, a combining prism or the like, and the combined light is projected to a screen by the projection optical system.
An optical device that uses a liquid crystal device as a display device usually utilizes a polarization and anisotropy of refractive index. Therefore, an unnecessary phase characteristic is added to a polarized light ray entering the liquid crystal display device surface with an angle. For this reason, for example, it is known that even if the liquid crystal display device is in a black display state, light leakage occurs so that a part of light is projected to the screen. In the liquid crystal image display apparatus (hereinafter referred to also as liquid crystal projector), such light significantly decreases contrast of a display image.
It is supposed that an optical axis of a liquid crystal molecule is directed to the normal direction of the liquid crystal display device surface in the black display state of the liquid crystal display device. Actually, the optical axis of the liquid crystal display device is not perpendicular to the device surface but has an angle (pretilt angle). However, the pretilt angle of the liquid crystal molecule is so small that the influence thereof can be neglected. The liquid crystal display device does not show birefringent property with respect to a polarized light ray entering the device at a right angle, and hence the reflected polarized light is not modulated in phase. Therefore, the light returns to the polarization beam splitter in the same state as the state of when the light has entered the device, and hence no leakage light occurs. However, the liquid crystal display device exhibits the birefringent property to a polarized light ray entering at a certain angle (incident angle) with respect to the normal of the surface in accordance with the incident angle. Therefore, the polarized light ray is modulated in phase. The polarized light after the phase modulation cannot be separated sufficiently by the polarization beam splitter so that the leakage light occurs.
It is known that the leakage light resulting from the above-mentioned cause may be suppressed by disposing a phase plate having refractive index anisotropy that is opposite to the refractive index anisotropy of the liquid crystal molecules at a vicinity of the liquid crystal display device. For instance, it is supposed that liquid crystal molecules in the liquid crystal display device have positive anisotropy. If the positive anisotropy is expressed by a refractive index ellipsoid, the positive anisotropy can be expressed as an ellipsoid having a large refractive index in the z axis direction (nz>nx=ny) as illustrated in FIG. 2. The z axis direction in FIG. 2 represents the optical axis direction. With respect to the medium having this anisotropy, a negative anisotropy phase plate, which has small refractive index anisotropy in the optical axis direction (nz<nx=ny), is disposed at a vicinity of the liquid crystal display device. The phase plate having negative refractive index anisotropy can effectively cancel a phase variation that occurs in the liquid crystal molecules, and as a result, the leakage light can be reduced.
There is widely known as this phase plate the one made of monocrystalline sapphire, anisotropy crystal such as quartz, or organic films extended and laminated. In addition, Japanese Patent Application Laid-Open No. 2004-102200 proposes a method of disposing a structural birefringence body, in which a high refractive index thin film and a low refractive index thin film are laminated, between the liquid crystal display device and a polarizer and/or between the liquid crystal display device and an analyzer in the liquid crystal projector. In addition, Japanese Patent Application Laid-Open No. 2006-39135 proposes a compensation method using a combination device of a phase plate formed of a structural birefringence body in which thin films having different refractive indexes are laminated and a wavelength plate formed of a one-dimensional periodical structure having a period of light wavelength or smaller in the in-plane direction. This method proposes to use a single device for correcting the leakage light that occurs in the light ray entering the liquid crystal display device at a right angle and the leakage light due to the oblique incident light ray.
Both Japanese Patent Application Laid-Open No. 2004-102200 and Japanese Patent Application Laid-Open No. 2006-39135 use the structural birefringence as the phase plate. In a fine structure of a light wavelength or smaller, the refractive index of the entire structure can be handled as an effective refractive index. For instance, in the case of the structure in which thin films of a refractive index nA and a film thickness LA, and thin films of a refractive index nB and a film thickness LB are repeated, it is known that the effective refractive index nTE with respect to the light polarized in the in-plane direction of the entire laminated thin film and the effective refractive index nTM with respect to the light polarized in the surface normal direction are expressed by the following equations.
            in      ⁢              -            ⁢      plane      ⁢                          ⁢      direction      ⁢              :            ⁢                          ⁢              n        TE              =                                                      L              A                        ⁢                          n              A              2                                +                                    L              B                        ⁢                          n              B              2                                                            L            A                    +                      L            B                                          surface      ⁢                          ⁢      normal      ⁢                          ⁢      direction      ⁢              :            ⁢                          ⁢              n        TM              =                                        L            A                    +                      L            B                                                              L              A                        /                          n              A              2                                +                                    L              B                        ⁢                          n              B              2                                          
In this case, “nTE>nTM” holds regardless of parameters of the refractive index and the film thickness, and the structural birefringence body acts as a negative phase plate having a small refractive index with respect to the light polarized in the surface normal direction.
The phase plate using the structural birefringence body has advantages that the phase characteristics can be controlled by the structure and that a phase plate with little deterioration with respect to heat and ultraviolet rays can be obtained by using inorganic material.
There is another leakage light due to the polarization beam splitter in addition to the leakage light due to the liquid crystal display device in the reflection type liquid crystal image display apparatus having the polarization beam splitter. As a method of compensating for the leakage light, Japanese Patent Publication No. 7-38050 proposes a method of disposing a ¼ wavelength plate between the polarization beam splitter and the liquid crystal display device.
The phase characteristics of the phase plate are determined by the product Δn·d of the refractive index anisotropy Δn and a total film thickness d. Approximately 1 to several microns of the total film thickness d is necessary in many cases even for the structural birefringence body that can obtain large anisotropy, and it is necessary to laminate multiple layers corresponding to the film thickness in number in order to obtain the desired phase characteristic. However, thin films are laminated for the phase plate formed of the structural birefringence body thin film in order to avoid an interaction region. For instance, Japanese Patent Application Laid-Open No. 2004-102200 describes an example of the structural birefringence body in which thin films of TiO2 and SiO2 are laminated to have a film thickness of 15 nm. In order to obtain a negative phase plate of retardance Δn·d=250 nm of the structural birefringence body thin film, it is necessary to laminate approximately 90 layers of thin films based on the theory of effective refractive index. Thus, the phase plate formed of the laminated birefringence body thin films has a disadvantage that a film thickness of each layer is so small that a large number of layers are necessary, resulting in a problem of manufacturing cost.
In addition, the methods of Japanese Patent Application Laid-Open No. 2004-102200 and Japanese Patent Application Laid-Open No. 2006-39135 can compensate for the leakage light due to the liquid crystal display device, but cannot compensate for a leakage light component due to the polarization beam splitter. Similarly, the method of Japanese Patent Publication No. 7-38050 cannot compensate for leakage light due to the liquid crystal display device.